1. Field of the Invention
The present invention relates to a two step method of resetting magnetizations of a spin valve read head and, more particularly, to, first, applying a first magnetic field for resetting the magnetization of a pinned layer and, optionally, the orientation of magnetic spins of a pinning layer, and, second, applying a second magnetic field for resetting the magnetization of biasing layers of the head.
2. Description of the Related Art
A read head includes a spin valve sensor for sensing magnetic fields on moving magnetic media, such as magnetic disks or magnetic tapes. The sensor includes a nonmagnetic conductive layer, hereinafter a "spacer layer", sandwiched between first and second ferromagnetic layers, hereinafter a "pinned layer", and a "free layer", respectively. Hard biasing layers are connected to opposite end portions of the free layer for stabilizing the magnetic moment of the free layer in a single domain state. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned at about 90.degree. to the magnetization of the free layer, and the magnetization of the free layer is free to respond to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer, hereinafter a "pinning layer".
Preferably, the thickness of the spacer layer is less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by interfaces between the spacer layer and the pinned and free layers. When the magnetizations of the pinned and free layers are substantially parallel, scattering is minimal and the electrical resistance of the sensor is at a minimum. When the magnetizations of the pinned and free layers are substantially antiparallel, scattering is maximized and the electrical resistance of the sensor is at a maximum. Changes in scattering alter the electrical resistance of the spin valve sensor in proportion to sin .theta., where .theta. is the angle between the magnetizations of the pinned and free layers. A spin valve sensor is characterized by a magnetoresistive (MR) coefficient (the ratio of the change in electrical resistance of the sensor to its maximum electrical resistance) that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a "giant magnetoresistive" (GMR) sensor.
A read head employing a spin valve sensor (hereinafter, a "spin valve read head") may be combined with an inductive write head to form a combined magnetic head. In a magnetic disk drive, an air bearing surface (ABS) of a combined magnetic head is supported adjacent a rotating disk to write information on or read information from the disk. Information is written to the rotating disk by magnetic fields which fringe across a gap between the first and second pole pieces of the write head. In a read mode, the electrical resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields on the rotating disk. When a sense current I.sub.S is conducted through the spin valve sensor, electrical resistance changes cause potential changes that are detected and processed as playback signals.
Another type of spin valve sensor, an antiparallel (AP) pinned spin valve sensor, is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin, which is incorporated into this application by this reference. The AP pinned spin valve sensor differs from the pinned layer spin valve sensor, described above, in that the pinned layer of the AP pinned spin valve sensor comprises multiple thin films, which are collectively referred to as an antiparallel (AP) pinned layer, while the pinned layer of the pinned layer spin valve sensor is a single thin film layer. The AP pinned layer has a nonmagnetic spacer film, hereinafter referred to as an antiparallel (AP) coupling film, sandwiched between first and second ferromagnetic thin films. The first thin film is exchange coupled to the pinning layer by being immediately adjacent thereto, and has its magnetic moment directed in a first direction. The second thin film is immediately adjacent to the free layer and is exchange coupled to the first thin film by the minimal thickness (in the order of 5 .ANG.) of the AP coupling film between the first and second thin films. The magnetic moment of the second thin film is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first film. The magnetic moments of the first and second films subtractively combine to provide a net pinning moment of the pinned layer. The direction of the net moment is determined by the thicker of the first and second thin films. The thicknesses of the first and second thin films are chosen to reduce the net moment. A reduced net moment results in a reduced demagnetization (demag) field from the AP pinned layer. Since the exchange coupling between the pinning layer and first thin film is inversely proportional to the net pinning moment, the exchange coupling is increased.
A transfer curve (a plot of the readback signal of the spin valve head versus the applied signal from the magnetic disk) of a spin valve sensor is defined by sin .theta.. A substantially flat portion of the transfer curve is selected for location of a bias point so that response of the sensor is substantially linear. Since positive and negative magnetic fields from a moving magnetic disk are typically equal in magnitude, it is important that positive and negative changes in the magnetoresistance of the spin valve sensor about the bias point on the transfer curve also be equal, which is referred to herein as read signal symmetry. The location of the bias point on the transfer curve is influenced by various magnetic fields when the sensor is in a quiescent state (sense current conducted, but an absence of magnetic fields from the rotating disk). When these magnetic fields are not balanced there will be read signal asymmetry in a positive or a negative direction with respect to the bias point.
A high performance spin valve head has high magnitude read signal output, and low, or no, read signal asymmetry. Where there is no read signal asymmetry, the transfer curve of the read signal is centered about a zero bias point. This means that from a point where the input signal is zero, the amplitudes of the positive and negative read signal outputs are equal as the input signals go between positive to negative levels. The level of performance of the spin valve head is dependent upon proper orientation and magnetizations of the aforementioned layers. If either of the pinned or biasing layers acquires a multi-magnetic domain state, read signal output will be decreased and read signal asymmetry will be increased. The impact on read signal output and read signal asymmetry will be even greater if the magnetic spins of the pinning layer are disoriented.
After fabrication of rows and columns of magnetic heads at a wafer level, the orientation of the magnetic spins of the pinning layers and the magnetic moments of the pinned and biasing layers are set by the application of magnetic fields in the presence of heat at or above the blocking temperature of the pinning layers. After setting the layers the heads typically undergo testing at the wafer level. The testing may be implemented by connecting test circuitry to terminals that are connected to the heads. The wafer is then diced into rows of sliders which contain the magnetic heads and then the row is lapped for forming the air bearing surfaces of the sliders. Unfortunately, after testing at the wafer level, dicing the heads into rows and lapping, the pinned and biasing layers may be changed from a single magnetic domain state to a multi magnetic domain state. In some heads, the orientation of the spins of the pinning layer may also be disturbed. Either or both of these conditions will cause a decrease in the performance of the spin valve head.